Richard Cary affirms that "A. F." is Annie Fields (1834-1915).
Cary says, the "wife of the publisher James T. Fields, earned
her own fame as poet and biographer. After Fields's death in
1881, Miss Jewett and Mrs. Fields became inseparable companions,
visiting extensively at each others' homes, and traveling
together in the United States, on four European tours, and a
Caribbean cruise" (Cary, Sarah Orne Jewett Letters, p.
47).